SPDT switch and communication unit using the same

ABSTRACT

An SPDT switch used for a communication unit includes first and second terminals, a common terminal, and first and second FETs with Schottky connection gates. The drain of the first FET is connected to the first terminal, and the source of the second FET is connected to the second terminal. The source of the first FET and the drain of the second FET are directly connected to each other, and are then connected to the common terminal. The pinch-off voltage V p1  of the first FET is set to satisfy 0 &gt;V p1 &gt;α−γ, and the pinch-off voltage V p2  of the second FET is set to satisfy 0 &gt;V p2 &gt;γ−β, where α&lt;γ&lt;β. A fixed potential γ is applied to the gate of the second FET, and one of potentials α and β is applied to the gate of the first FET, so that one of the first and second terminals can be electrically connected to the common terminal. Additional resonance and/or switching elements may be included as well.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an SPDT (single pole double throw) switch, and a communication unit using the same. More particularly, the present invention relates to an SPDT switch for use in a mobile communication unit as an antenna switch, and to a communication unit using the same.

2. Description of the Related Art

Recent demands to reduce the size and cost of mobile communication units require that single pole double throw (SPDT) switches used as antenna switches be reduced in size and cost. An SPDT switch is a switch having three terminals, one of which is connectable to either of the other two terminals.

FIG. 7 is a circuit diagram showing a conventional SPDT switch disclosed in Japanese Unexamined Patent Application Publication No. 9-23101.

Referring to FIG. 7, an SPDT switch 1 includes a first terminal P₁, a second terminal P₂, a common terminal P₃, a first field-effect transistor (FET) Q₁, a second FET Q₂, a first inductor L₁, a second inductor L₂, resistors R₁, R₂, and R₃, a first control terminal P₄, a second control terminal P₅, and a third control terminal P₆. The source of the first FET Q₁ is connected to the first terminal P_(l), and the source of the second FET Q₂ is connected to the second terminal P₂. The drain of the first FET Q₁ and the drain of the second FET Q₂ are connected to each other, and are connected to the common terminal P₃. The first inductor L₁ is connected across the drain and source of the first FET Q₁, and the second inductor L₂ is connected across the drain and source of the second FET Q₂. The gate of the first FET Q₁ is connected to the first control terminal P₄ via the resistor R₁, and the gate of the second FET Q₂ is connected to the second control terminal P₅ via the resistor R₂. The drain of the first FET Q₁ and the drain of the second FET Q₂ are connected to the third control terminal P₆ via the resistor R₃. Each of the first FET Q₁ and the second FET Q₂ has a pinch-off voltage set at −0.5 V. Symbol “D” in FIG. 7 represents the drain.

In the SPDT switch 1 having such a construction, potentials of 0 V, 0 V, and −3 V are applied to the first control terminal P₄, the second control terminal P₅, and the third control terminal P₆, respectively. Then, the first FET Q₁ has a potential of 0 V at the drain and source, and the gate-drain (or gate-source) voltage is 0 V, thereby turning on the first FET Q₁. The second FET Q₂ also has a potential of 0 V at the drain and source, and the gate-drain (or gate-source) voltage is −3 V, which is less than the pinch-off voltage, thus turning off the second FET Q₂. In the off state, the second FET Q₂ has an off-capacitance across the drain and source. The inductance of the second inductor L₂ is set so that the second inductor L₂ may form a parallel resonance with the off-capacitance of the second FET Q₂ having a resonant frequency synchronous with the frequency of an undesired signal. In theory, infinite impedance is thus obtained across the drain and source of the second FET Q₂ at the frequency of such an undesired signal. Therefore, an electrical connection is established between the first terminal P₁ and the common terminal P₃ via the first FET Q₁, and no electrical connection occurs between the second terminal P₂ and the common terminal P₃ because infinite impedance is obtained at the parallel resonance between the off-capacitance of the second FET Q₂ and the second inductor L₂.

On the other hand, suppose that potentials of −3 V, 0 V, and 0 V are applied to the first control terminal P₄, the second control terminal P₅, and the third control terminal P₆, respectively. In contrast to the previous description, an electrical connection between the second terminal P₂ and the common terminal P₃ is established via the second FET Q₂, and no electrical connection occurs between the first terminal P₁ and the common terminal P₃ because infinite impedance is obtained by the parallel resonance between the off-capacitance of the first FET Q₁ and the first inductor L₁.

The SPDT switch 1 therefore allows either one of the first terminal P₁ and the second terminal P₂ to be electrically connected to the common terminal P₃ by changing the voltages to be applied to the first control terminal P₄ and to the second control terminal P₅.

However, the SPDT switch 1 shown in FIG. 7 is disadvantageous in that two potentials of 0 V and −3 V must be alternately applied to the first control terminal P₄ and the second control terminal P₅. In other words, while 0 V or −3 V is applied to the first control terminal P₄, −3 V or 0 V must be simultaneously applied to the second control terminal P₅. Specifically, two control lines adapted to change the potentials to be applied to the respective terminals are required, or otherwise, a single control line branched into two and configured so that either signal may be inverted is required.

In such cases, an increased area may be required for such a control line(s). Otherwise, an extra control port such as a CPU (central processing unit) or logic for allowing a control signal to be inverted may be required, thus, making it difficult to reduce the size and cost of the switch.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides an SPDT switch which can be easily controlled and which is compact, and a communication unit using the same.

To this end, in an aspect of the present invention, an SPDT switch includes first and second terminals, and first and second FETs with Schottky connection gates. The drain of the first FET and the source of the second FET are connected to the first terminal and the second terminal, respectively, and the source of the first FET and the drain of the second FET are connected to the common terminal. A fixed potential γ is applied to the gate of the second FET, and one of potentials α and β is applied to the gate of the first FET, where α<γ<β, to allow one of the first and second terminals to be electrically connected to the common terminal. The pinch-off voltage V_(p1) of the first FET is set to satisfy 0>V_(p1)>α−γ, and the pinch-off voltage V_(p2) of the second FET is set to satisfy 0>V_(p2)>γ−β.

The SPDT switch may further include a first inductor connected in parallel to the first FET, and a second inductor connected in parallel to the second FET1.

The SPDT switch may further include a first inductor connected in series to the first FET, a first capacitor connected in parallel to the series connection of the first FET and the first inductor, a second inductor connected in series to the second FET, and a second capacitor connected in parallel to the series connection of the second FET and the second inductor.

The SPDT switch may further include a third FET with a Schottky connection gate, having a pinch-off voltage V_(p3) set to satisfy 0>V_(p3)>γ−β, and a fourth FET with a Schottky connection gate, having a pinch-off voltage V_(p4) set to satisfy 0>V_(p4)>α−γ. The third FET may have a drain connected to the drain of the first FET, a source connected to ground via a first ground capacitor, and a gate connected to the gate of the second FET. The fourth FET may have a drain connected to the source of the second FET, and a source connected to ground via a second ground capacitor, and a gate connected to the gate of the first FET.

In another aspect of the present invention, a communication unit contains an SPDT switch such as that described above.

An SPDT switch embodied by the present invention can be easily controlled and reduced in size and cost. Furthermore, a communication unit using such an SPDT switch can also be reduced in size and cost.

Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings, in which like references denote like elements and parts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an SPDT switch according to an embodiment of the present invention;

Fig. 2 is an equivalent circuit diagram of the SPDT switch shown in FIG. 1;

FIG. 3 is a circuit diagram of a modification of the SPDT switch according to the present invention;

FIG. 4 is a circuit diagram of another modification of the SPDT switch according to the present invention;

FIG. 5 is a circuit diagram of still another modification of the SPDT switch according to the present invention

FIG. 6 is a block diagram of a communication unit according to an embodiment of the present invention; and

FIG. 7 is a circuit diagram of a conventional SPDT switch.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 shows an SPDT switch according to an embodiment of the present invention. In FIG. 1, the same or like reference numerals are used to designate the same or like elements and components as in FIG. 7.

Referring to FIG. 1, an SPDT switch 10 includes a first terminal P₁, a second terminal P₂, a common terminal P₃, a first field-effect transistor (FET) Q₃ having a Schottky connection gate, a second FET Q₄ having a Schottky connection gate, capacitors C₁, C₂, and C₃, a first control terminal P₇, and a second control terminal P₈. The drain of the first FET Q₃ is connected to the first terminal P₁ via the capacitor C₁, and the source of the second FET Q₄ is connected to the second terminal P₂ via the capacitor C₂. The source of the first FET Q₃ and the drain of the second FET Q₄ are directly connected to each other, and are then connected to the common terminal P₃ via the capacitor C₃. The gate of the first FET Q₃ is connected to the first control terminal P₇, and the gate of the second FET Q₄ is connected to the second control terminal P₈. The capacitors C₁, C₂, and C₃ are coupling capacitors each having low impedance at a signal frequency.

Now, suppose there are three potentials α, β, and γ for the operation of the SPDT switch 10, where α<γ<β. Furthermore, suppose the pinch-off voltage V_(p1) of the first FET Q₃ is set to satisfy 0>V_(p1)>α−γ, and the pinch-off voltage V_(p2) of the second FET Q₄ is set to satisfy 0>V_(p2)>γ−β. As a specific example, suppose that α=0 V, γ=3 V, and β=6 V. Then, the pinch-off voltage V_(p1) of the first FET Q₃ is in the range of 0>V_(p1)>−3 V, and the pinch-off voltage V_(p2) of the second FET Q₄ is in the range of 0>V_(p2)>−3 V. Now, the pinch-off voltages V_(p1) and V_(p2) are set at V_(p1)=V_(p2)=−0.5 V.

In operation, a potential of 3 V (γ) is first applied to the second control terminal P₈. Then, a potential of 0 V (α) is applied to the first control terminal P₇. Since the source of the first FET Q₃ and the drain of the second FET Q₄ are directly connected to each other, and the gate potential (3 V) of the second FET Q₄ is higher than the gate potential (0 V) of the first FET Q₃, a potential of 3V, which is equal to the gate potential of the second FET Q₄, is obtained at the source of the first FET Q₃ and at the drain of the second FET Q₄. The drain of the first FET Q₃, which is open to a direct current, has a potential of 0 V equal to the associated gate potential. The source of the second FET Q₄, which is also open to a direct current, has a potential of 3 V equal to the associated gate potential. Therefore, the gate-source voltage of the first FET Q₃ is −3 V. Since the first FET Q₃ has a pinch-off voltage V_(p1)=−0.5 V, the first FET Q₃ is off. On the other hand, the gate-source (gate-drain) voltage of the second FET Q₄ is 0 V. Since the second FET Q₄ also has a pinch-off voltage V_(p2)=−0.5 V, the second FET Q₄ is on. Consequently, an electrical connection between the second terminal P₂ and the common terminal P₃ is established at the signal frequency while no electrical connection occurs between the first terminal P₁ and the common terminal P₃ at the signal frequency.

For clarification, FIG. 2 shows an equivalent circuit of the first and second FETs Q₃ and Q₄ using diodes, which illustrates the potential at each of the electrodes of the first and second FETs Q₃ and Q₄.

As depicted in FIG. 2, each FET having a Schottky connection gate is equivalent to a combination of two diodes having anodes which are connected together and become the gate, and cathodes which become the drain and the source. When a potential of 0 V is applied to the first control terminal P₇, the drain of the first FET Q₃ also has a potential of 0 V equal to the gate potential. When a potential of 3 V is applied to the second control terminal P₈, the source of the second FET Q₄ also has a potential of 3 V equal to the gate potential. The potentials of the source of the first FET Q₃ and the drain of the second FET Q₄, of which the corresponding diodes are supplied with 0 V and 3 V, respectively, are 3 V which is equal to the higher gate potential.

Referring again to FIG. 1, as an alternative, a potential of 6 (β) may be applied to the first control terminal P₇, while a potential of 3 V (γ) is still applied to the second control terminal P₈. In this case, since the gate potential (6 V) of the first FET Q₃ is higher than the gate potential (3 V) of the drain and source of the first FET Q₃, and at the drain of the second FET Q₄, which is equal to the gate potential of the first FET Q₃. It will be anticipated that the source of the second FET Q₄ still has a potential of 3 V equal to the associated gate potential. Therefore, the gate-source (gate-drain) voltage of the first FET Q₃ is 0 V. Since the first FET Q₃ still has a pinch-off voltage V_(p1)=−0.5 V, the first FET Q₃ is on. On the other hand, the gate-drain voltage of the second FET Q₄ is −3 V. Since the second FET Q₄ also has a pinch-off voltage V_(p2)=−0.5 V, the second FET Q₄ is off. Consequently, in contrast to the above, an electrical connection between the first terminal P₁ and the common terminal P₃ is established at the signal frequency while no electrical connection occurs between the second terminal P₂ and the common terminal P₃ at the signal frequency.

In this way, while the potential to be applied to the control terminal P₈ is fixed to 3 V (γ), one of potentials of 0 V (α) and 6 V (β) is applied to the first control terminal P₇, so that the SPDT switch 10 allows either one of the first terminal P₁ and the second terminal P₂ to be electrically connected to the common terminal P₃.

As a result, the SPDT switch 10 only requires a control circuit by which voltages are alternatively applied to either one of the control terminals. This can make an area required for control lines smaller than in the conventional SPDT switch which requires certain potentials to be alternatively applied to two control terminals. Furthermore, the switch 10 only requires one control port such as a CPU, and logic for allowing a control signal to be inverted is no longer necessary. Therefore, an SPDT switch having reduced size and cost and which can be easily controlled is attained.

In the SPDT switch 10, when in the off state, the first FET Q₃ and the second FET Q₄ still have off-capacitances across the drains and sources, although they are small. Even when the first FET Q₃ or the second FET Q₄ is off, signals will thus slightly flow through the off-capacitance, which might decrease the isolation.

FIG. 3 shows a modification of the SPDT switch according the present invention. In FIG. 3, the same or like reference numerals are given to the same or like elements or components as in FIG. 1, and thus the description thereof is omitted.

Referring to FIG. 3, an SPDT switch 20 further includes a first inductor L₃ connected across the drain and source of the first FET Q₃, and a second inductor L₄ connected across the drain and source of the second FET Q₄. The inductance of the first inductor L₃ is set so that the first inductor L₃ may form a parallel resonance with the off-capacitance of the first FET Q₃ having a resonant frequency synchronous with the frequency of an undesired signal. The inductance of the second inductor L₄ is set so that the second inductor L₄ may form a parallel resonance with the off-capacitance of the second FET Q₄ having a resonant frequency synchronous with the frequency of an undesired signal.

In the SPDT switch 20 having such a construction, the impedance is extremely low when the FETs are on. When the FETs are off, substantially infinite impedance is obtained by parallel resonance between the off-capacitance and the associated inductors.

Specifically, when the first FET Q₃ is on, the first terminal P₁ is electrically connected to the common terminal P₃ via the first FET Q₃. No electrical connection occurs between the second terminal P₂ and the common terminal P₃ due to infinite impedance by the parallel resonance between the off-capacitance of the second FET Q₄ and the second inductor L₄.

When the second FET Q₄ is on, the second terminal P₂ is electrically connected to the common terminal P₃ via the second FET Q₄. No electrical connection occurs between the first terminal P₁ and the common terminal P₃ due to infinite impedance by the parallel resonance between the off-capacitance of the first FET Q₃ and the first inductor L₃.

Therefore, the decreased isolation due to leakage of signals through the off-capacitance of the first FET Q₃ or the second FET Q₄ can be prevented.

In the SPDT switch 10 or 20 described in conjunction with FIGS. 1 to 3, signals are passed across the drain and source of the first FET Q₃ when the first FET Q₁ is on, and signals are passed across the drain and source of the second FET Q₄ when the second FET Q₄ is on. However, the FETs still have an extremely low resistance across the drain and source in the on states, which might cause signal loss.

FIG. 4 shows a modification of the SPDT switch according to the present invention. In FIG. 4, the same or like reference numerals are given to the same or like elements or components as in FIG. 1, and thus the description thereof is omitted.

Referring to FIG. 4, an SPDT switch 30 further includes a first inductor L₅ connected in series to the source of the first FET Q₃, and a second inductor L₆ connected in series to the drain of the second FET Q₄. The SPDT switch 30 also includes a first capacitor C₄ connected in parallel to the series connection of the first FET Q₃ and the first inductor L₅, and a second capacitor C₅ connected in parallel to the series connection of the second FET Q₄ and the second inductor L₆. The inductance of the first inductor L₅ is set so that the first inductor L₅ may form a series resonance with the off-capacitance of the first FET Q₃ having a resonant frequency synchronous with the frequency of a desired signal. The capacitance of the first capacitor C₄ is set to coincide with the off-capacitance of the first FET Q₃, and is also set so that the first capacitor C₄ may form a parallel resonance with the inductor L₅ having a resonant frequency synchronous with the frequency of an undesired signal. The inductance of the second inductor L₆ is set so that the second inductor L₆ may form a series resonance with the off-capacitance of the second FET Q₄ having a resonant frequency synchronous with the frequency of a desired signal. The capacitance of the second capacitor C₅ is set to coincide with the off-capacitance of the second FET Q₄, and is also set so that the second capacitor C₅ may form a parallel resonance with the second inductor L₆ having a resonant frequency synchronous with the frequency of an undesired signal.

When the first FET Q₃ is on, parallel resonance occurs between the first capacitor C₄ and the first inductor L₅, and series resonance occurs between the off-capacitance of the second FET Q₄ and the second inductor L₆.

When the second FET Q₄ is on, parallel resonance occurs between the second capacitor C₃ and the second inductor L₅, and series resonance occurs between the off-capacitance of the first FET Q₃ and the first inductor L₅.

Impedance generated by the series resonance is substantially absent while impedance generated by the parallel resonance is substantially infinite. Therefore, the switch 30 is different from the switches 10 and 20 shown in FIGS. 1 to 3 in this point.

Specifically, when the first FET Q₃ is off, an electrical connection is established between the first terminal P₁ and the common terminal P₃ because of the substantial absence of impedance generated by the series resonance between the off-capacitance of the first FET Q₃ and the first inductor L₅. Then, no electrical connection occurs between the second terminal P₂ and the common terminal P₃ because impedance generated by the parallel resonance between the second capacitor C₅ and the second inductor L₆ is substantially infinite.

When the second FET Q₄ is off, an electrical connection is established between the second terminal P₂ and the common terminal P₃ because of the substantial absence of impedance generated by the series resonance between the off-capacitance of the second FET Q₄ and the second inductor L₆. Then, no electrical connection occurs between the first terminal P₁ and the common terminal P₃ because impedance generated by the parallel resonance between the first capacitance C₄ and the first inductor L₅ is substantially infinite.

Since impedance when an electrical connection is established is substantially absent, because of the use of series resonance between the off-capacitance of the FETs and the associated inductors, the signal loss during electrical connection can be reduced.

FIG. 5 shows another modification of the SPDT switch according to the present invention. In FIG. 5, the same or like reference numerals are given to the same or like elements or components as in FIG. 1, and thus the description thereof is omitted.

Referring to FIG. 5, an SPDT switch 40 additionally includes a third FET Q₅ having a Schottky connection gate, a fourth FET Q₆ having a Schottky connection gate, a first ground capacitor C₆, and a second ground capacitor C₇. The third FET Q₅ has a drain connected to the drain of the first FET Q₃ and a source connected to ground via the first ground capacitor C₆. The gate of the third FET Q₅ is connected to the gate of the second FET Q₄. The fourth FET Q₆ has a drain connected to the source of the second FET Q₄ and a source connected to ground via the second ground capacitor C₇. The gate of the fourth FET Q₆ is connected to the gate of the first FET Q₃. The pinch-off voltage V_(p3) is set to satisfy 0>V_(p3)>γ−β, as is the pinch-off voltage V_(p2) of the second FET Q₄, and the pinch-off voltage V_(p4) of the fourth FET Q₆ is set to satisfy 0>V_(p4)>α−γ, as is the pinch-off voltage V_(p1) of the first FET Q₃. Also, V_(p3)=V_(p4)=−0.5 V may be set, which corresponds to the pinch-off voltages V_(p1) and V_(p2) of the first FET Q₃ and the second FET Q₄ in the SPDT switch 10 shown in FIGS. 1 and 2. The first and second ground capacitors C₆ and C₇ are coupling capacitors each having low impedance at signal frequencies.

In the SPDT switch 40 having such a construction, supposing that a potential of 3 V (γ) is applied to the second control terminal P₈ and a potential of 0 V (α) is applied to the first control terminal P₇, the first FET Q₃ has potentials of 0 V, 3 V, and 3 V at the gate, the drain, and the source, respectively, and is turned off. The second FET Q₄ has potentials of 3 V, 3 V, and 3 V at the gate, the drain, and the source, respectively, and is turned on. The third FET Q₅ has potentials of 3 V, 3 V, and 3 V at the gate, the drain, and the source, respectively, and is turned on. The fourth FET Q₆ has potentials of 0 V, 3 V, and 0 V at the gate, the drain, and the source, respectively, and is turned off. In short, the first and fourth FETs Q₃ and Q₆ are off, and the second and third FETs Q₄ and Q₅ are on. In this context, an electrical connection is established between the second terminal P₂ and the common terminal P₃ since the second FET Q₄ is on, and no electrical connection occurs between the first terminal P₁ and the common terminal P₃ since the first FET Q₃ is off. Since the third FET Q₅ is in conduction, the drain of the third FET Q₅ or the drain of the first FET Q₃ is grounded at high frequencies. This inhibits signals from passing between the first terminal P₁ and the common terminal P₃ through the off-capacitance of the first FET Q₃, thus increasing the isolation.

In turn, supposing that a potential of 3 V (γ) is applied to the second control terminal P₈ and a potential of 6 V (β) is applied to the first control terminal P₇, the first FET Q₃ has potentials of 6 V, 6 V, and 6 V at the gate, the drain, and the source, respectively, and is turned on. The second FET Q₄ has potentials of 3 V, 6 V, and 6 V at the gate, the drain, and the source, respectively, and is turned off. The third FET Q₅ has potentials of 3 V, 6 V, and 3 V at the gate, the drain, and the source, respectively, and is turned off. The fourth FET Q₆ has potentials of 6 V, 6 V, and 6 V at the gate, the drain, and the source, respectively, and is turned on. In short, the first FET Q₃ and the fourth FET Q₆ are on, and the second FET Q₄ and the third FET Q₅ are off. In this context, an electrical connection is established between the first terminal P₁ and the common terminal P₃ since the first FET Q₃ is on, and no electrical connection occurs between the second terminal P₂ and the common terminal P₃ since the FET Q₄ is off. Since the fourth FET Q₆ is in conduction, the drain of the fourth FET Q₆ or the source of the second FET Q₄ is grounded at high frequencies. This inhibits signals from passing between the second terminal P₂ and the common terminal P₃ through the off-capacitance of the second FET Q₄, thus increasing the isolation.

Accordingly, the SPDT switch 40 is advantageous in that the isolation is increased.

Typically, in FETs having Schottky connection gates, drains and sources are substantially symmetrical in structure with respect to the gates, and drains and sources may be used interchangeably. In the illustrated embodiment, drains and sources of the FETs are not necessarily fixed, but may be interchangeable, attaining similar operational advantages. It will be appreciated that the connections of the drains and the sources of the FETs in the present invention are not exhaustive and other types may also fall within the scope of the invention.

Since the drains and the sources of FETs may be interchanged, the present invention is not limited to the case in which a certain fixed potential is applied to the second control terminal P₈ and one of two potentials is applied to the first control terminal P₇. A certain fixed potential may also be applied to the first control terminal P₇ while one of two potentials is applied to the second control terminal P₈. Either case provides the same operational advantages.

As will be anticipated from the foregoing embodiment, both the first and second control terminals P₇ and P₈ are connected to the gates of the first and second FETs Q₃ and Q₄, respectively, and no current flows. Potentials to be applied to FETs may be obtained by an approach in which more than one DC power supply are connected in series so that potentials α, γ, and β may be obtained. In another approach, voltages may be divided by resistance from a DC power supply having a potential exceeding β to obtain the potentials α, γ, and β.

FIG. 6 is a block diagram showing a communication unit in accordance with an embodiment of the present invention.

Referring to FIG. 6, a communication unit 50 includes an antenna 51, the SPDT switch 10 shown in FIG. 1, a transmitter circuit 52, a receiver circuit 53, and a control circuit 54. The antenna 51 is connected to the common terminal P₃ of the SPDT switch 10 employed as an antenna switch. The first terminal P₁ and second terminal P₂ of the SPDT switch 10 are connected to the transmitter circuit 52 and the receiver circuit 53, respectively. The transmitter circuit 52 and the receiver circuit 53 are connected to the control circuit 54, and the control circuit 54 is connected to the first control terminal P₇ of the SPDT switch 10. Although not shown, a predetermined fixed potential is applied to the second control terminal P₈ of the SPDT switch 10.

The control circuit 54 in the communication unit 50 switches the potentials to be applied to the first control terminal P₇ of the SPDT switch 10 so that the antenna 51 may be coupled to the transmitter circuit 52 for transmission or coupled to the receiver circuit 53 for reception.

The SPDT switch 10 of the present invention, which is used as an antenna switch, can reduce the size and cost of the communication unit 50.

A communication unit typically requires less electrical power for reception than for transmission. The SPDT switch used as an antenna switch for a communication unit is thus set so that the FET through which transmission signals pass in its on state has a higher power tolerance than that of the FET through which reception signals pass in its on state. This can prevent over-specification of the SPDT switch, further reducing the cost of the overall communication unit.

While the communication unit 50 shown in FIG. 6 includes the SPDT switch 10, the SPDT switch 20, 30 or 40 shown in FIG. 3, 4 or 5 may be instead contained in the communication unit 50. Some of these SPDT switches may increase the isolation, and may reduce signal loss or electric power consumption while attaining the same operational advantages.

Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. Therefore, the present invention is not limited by the specific disclosure herein. 

What is claimed is:
 1. An SPDT switch comprising: a first terminal; a second terminal; a common terminal; and first and second FETs each having two respective main terminals and respective Schottky connection gates; wherein one main terminal of said first FET and one main terminal of said second FET are connected to said first terminal and said second terminal, respectively; the other main t terminal of said first FET and the other main terminal of said second FET are connected to said common terminal; a fixed potential γ is applied to the gate of said second FET, and one of potentials α and β are applied to the gate of said first FET without changing the fixed potential applied to the gate of said second FET, where α<γ<β, to allow one of said first and second terminals to be electrically connected to said common terminal; and the pinch-off voltage 0>V_(p1)>α−γ, and the pinch-off voltage V_(p2) of said second FET is set to satisfy 0>V_(p2)>γ−β.
 2. An SPDT switch according to claim 1, further comprising: a first inductor connected in parallel to said first FET; and a second inductor connected in parallel to said second FET.
 3. An SPDT switch according to claim 1, further comprising: a first inductor connected in series to said first FET; a first capacitor connected in parallel to the series connection of said first FET and said first inductor; a second inductor connected in series to said second FET; and a second capacitor connected in parallel to the series connection of said second FET and said second inductor.
 4. An SPDT switch according to claim 1, further comprising: a third FET having two main terminals and a Schottky connection gate, having a pinch-off voltage V_(p3) set to satisfy 0>V_(p3)>γ−β; and a fourth FET having two main terminals and a Schottky connection gate, having a pinch-off voltage V_(p4) set to satisfy 0>V_(p4)>α−γ, wherein said third FET has one main terminal connected to said one main terminal of said first FET, the other main terminal connected to ground via a first ground capacitor, and a gate connected to the gate of said second FET; and said fourth FET has one main terminal connected to said one main terminal of said second FET, the other main terminal connected to ground via a second ground capacitor, and a gate connected to the gate of said first FET.
 5. A communication unit including an SPDT switch according to any of claims 1 to 4; a transmitter circuit connected to one of said first and second terminals; a receiver circuit connected to the other of said first and second terminals; and a control circuit connected to said gate of said first FET for applying thereto said one of said potentials α and β. 